Labeled alginate conjugates for molecular imaging applications

ABSTRACT

Described are bifunctional NOTA-based derivatives capable of conjugating with alginate and with metal ions, as well as NOTA-alginate conjugates which can be labeled with stable or radioactive metal ions. Also described are conjugation methods of the bifunctional NOTA-based linker with alginate, and methods of using radiometal-labeled NOTA-alginate conjugates or other radio-labeled alginate conjugates as imaging reagents.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. §119(e) to U.S.Provisional Application No. 61/568,796, filed Dec. 9, 2011, the entirecontent of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention generally relates to the field of alginateconjugates and their use as imaging reagents.

BACKGROUND

Cross-linked polymer hydrogel materials are widely utilized in thebiomedical industry. They are used in contact lenses, blood contactmaterials, controlled release formulations, wound dressings,bioadhesives, membranes, superabsorbents, cell encapsulation andimmunoisolation materials, and tissue engineering scaffolds. Among thedifferent polymers, the naturally occurring polysaccharide alginic acidhas found biomedical applications because of its biocompatibility,relative biological inertness, and the ability to engineer itsmechanical properties by introducing various types of chemical andphysical crosslinks. Alginic acid distinguishes itself from otherbiologically occurring polysaccharides in its ability to form stiffhydrogels when exposed to cross-linking calcium ions at slightlysupraphysiological concentrations. This property has been utilized todevise a treatment for damaged heart tissue of patients at risk foradverse remodeling of the left ventricle of the heart following acutemyocardial infarction (AMI). An aqueous soluble formulation of sodiumalginate and calcium-D-gluconate, the concentration of each componentcarefully chosen to achieve partial crosslinking of the alginatemolecules, yet providing for a stable free flowing liquid, is injectedinto the coronary artery of AMI patients after revascularization. Theformulation undergoes a transition from liquid to gel when in contactwith the infarcted cardiac tissue as a result of the elevatedextracellular calcium concentration in the reperfused cardiac tissue.The hydrogel then deposits in the interstitial tissue and exerts abeneficial therapeutic effect by reducing adverse remodeling and heartfailure, potentially because of its mechanical support of the weakenedheart wall.

The deposition of this alginate hydrogel in the injured reperfusedmyocardium of AMI patients is unknown as comprehensive invasive hearttissue sampling in human patients cannot be conducted. Also, the utilityof invasive tissue sampling techniques in a preclinical setting islimited because the surgical intervention often constitutes a terminalprocedure that prevents longitudinal assessment in the same researchsubject. Non-invasive imaging techniques can offer a solution byproviding this information without surgical intervention or terminalprocedures. Imaging modalities such as echocardiography, computedtomography, magnetic resonance imaging, and nuclear imaging such aspositron emission tomography (PET) and single photon emission computedtomography (SPECT) use specialized imaging reagents and/orinstrumentation to assess heart structure, function, perfusion andremodeling in patients with AMI or heart failure as well as in animalmodels of these diseases.

SUMMARY

One aspect of the present invention relates to a compound having theformula:

-   -   wherein X is —(CH₂)_(m)C(O)—, with m being 1, 2, or 3;    -   L is a linker selected from the group consisting of:

-   -   wherein n, n′ and n″ are each independently a number from 0 to        10; and    -   Y—H is selected from the group consisting of:

-   -   wherein R is selected from the group consisting of hydrogen,        alkyl, benzyl or an aromatic group;    -   or a pharmaceutically acceptable salt or solvate thereof.

In certain embodiments of this aspect, the compound further comprises astable or radioactive metal ion chelated by the1,4,7-triazacyclononane-1,4,7-triacetic acid moiety. According to one ormore embodiments, the stable or radioactive metal ion comprises agallium ion. In some embodiments, the gallium ion is one suitable forimaging, such as ⁶⁶Ga, ⁶⁷Ga or ⁶⁸Ga. In other embodiments, theradioactive metal ion is one suitable for imaging, such as ⁶⁰Cu, ⁶¹Cu,⁶²Cu, ⁶⁴Cu, ⁶⁷Cu, or ¹¹¹In.

Another aspect of the invention pertains to an alginate conjugate havingthe formula:

-   -   wherein X is —(CH₂)_(m)C(O)—, with m being 1, 2, or 3;    -   L is a linker selected from the group consisting of:

-   -   wherein n, n′ and n″ are each independently a number from 0 to        10;    -   Alg is alginic acid or an alginate salt; and    -   Y is a spacer directly attached to the reducing end unit of        alginate or the carboxyl groups of the alginate polymer chain        via the active nitrogen, and selected from the group consisting        of:

-   -   wherein R is selected from the group consisting of hydrogen,        alkyl, benzyl or an aromatic group;    -   or a pharmaceutically acceptable salt or solvate thereof.

In one or more embodiments, the alginate salt comprises a monovalentcation salt and/or a multivalent cation salt. The monovalent and/ormultivalent cation may comprise one or more of sodium, potassium,lithium, rubidium, cesium, ammonium, calcium, strontium, barium andmagnesium. In some embodiments, the alginate salt is sodium alginate,calcium alginate, or a mixture of sodium alginate and calcium alginate.

According to one or more embodiments of this aspect, the conjugatefurther comprises a stable or radioactive metal ion chelated by the1,4,7-triazacyclononane-1,4,7-triacetic acid moiety of the conjugate. Incertain embodiments, the stable or radioactive metal ion comprisesgallium ion. Some embodiments provide that the gallium ion is onesuitable for imaging, such as ⁶⁶Ga, ⁶⁷Ga or ⁶⁸Ga. In other embodiments,the radioactive metal ion comprises one suitable for imaging such as⁶⁰Cu, ⁶¹Cu, ⁶²Cu, ⁶⁴Cu, ⁶⁷Cu, or ¹¹¹In.

In one or more embodiments, the stable or radioactive metal ion ischelated by the 1,4,7-triazacyclononane-1,4,7-triacetic acid moiety ofthe conjugate at a temperature between 20° C. to 100° C.

Another aspect of the invention relates to a method of imaging in amammal comprising administering a radio-labeled alginate conjugate to amammal, and imaging the temporal and spatial distribution of theradio-labeled alginate conjugate. According to one or more embodimentsof this aspect, the alginate conjugate has the formula:

-   -   wherein X is —(CH₂)_(m)C(O)—, with m being 1, 2, or 3;    -   L is a linker selected from the group consisting of:

-   -   wherein n, n′ and n″ are each independently a number from 0 to        10;    -   Alg is alginic acid or an alginate salt; and    -   Y is a spacer directly attached to the reducing end unit of        alginate or the carboxyl groups of the alginate polymer chain        via the active nitrogen, and selected from the group consisting        of:

-   -   wherein R is selected from the group consisting of hydrogen,        alkyl, benzyl or an aromatic group;

or a pharmaceutically acceptable salt or solvate thereof, and thealginate conjugate further comprises a stable or radioactive metal ionchelated by the 1,4,7-triazacyclononane-1,4,7-triacetic acid moiety ofthe conjugate.

In one or more embodiments, the alginate salt comprises a monovalentcation salt and/or a multivalent cation salt. The monovalent and/ormultivalent cation may comprise one or more of sodium, potassium,lithium, rubidium, cesium, ammonium, calcium, strontium, barium andmagnesium. In some embodiments, the alginate salt is sodium alginate,calcium alginate, or a mixture of sodium alginate and calcium alginate.

In certain embodiments, the stable or radioactive metal ion comprises agallium ion. Some embodiments provide that the gallium ion is onesuitable for imaging, such as ⁶⁶Ga, ⁶⁷Ga or ⁶⁸Ga. In other embodiments,the radioactive metal ion comprises one suitable for imaging such as⁶⁰Cu, ⁶¹Cu, ⁶²Cu, ⁶⁴Cu, ⁶⁷Cu, or ¹¹¹In.

In other embodiments, the alginate is conjugated to an iodinatedtyramine or tyramine derivative. In some embodiments, the iodinatetyramine or tyramine derivative may have the following formula:

-   -   wherein each X is independently hydrogen or iodine and R and R′        are each independently hydrogen, alkyl, benzyl or an aromatic        group. In some embodiments, the iodine is selected from the        isotopes of ¹²³I, ¹²⁴I, ¹²⁵I and ¹³¹I.

In some embodiments of this method, the mammal is a human.

Another aspect of the invention relates to a method of imaging alginatedeposition in a mammal comprising administering a radio-labeled alginateconjugate to a mammal and imaging the radio-labeled alginate conjugate.In certain embodiments, the radio-labeled alginate conjugate ispartially cross-linked.

According to one or more embodiments of this aspect, the alginateconjugate has the formula:

-   -   wherein X is —(CH₂)_(m)C(O)—, with m being 1, 2, or 3;    -   L is a linker selected from the group consisting of:

-   -   wherein n, n′ and n″ are each independently a number from 0 to        10;    -   Alg is alginic acid or an alginate salt; and    -   Y is a spacer directly attached to the reducing end unit of        alginate or the carboxyl groups of the alginate polymer chain        via the active nitrogen, and selected from the group consisting        of:

-   -   wherein R is selected from the group consisting of hydrogen,        alkyl, benzyl or an aromatic group;

or a pharmaceutically acceptable salt or solvate thereof, and thealginate conjugate further comprises a stable or radioactive metal ionchelated by the 1,4,7-triazacyclononane-1,4,7-triacetic acid moiety ofthe conjugate.

In one or more embodiments, the alginate salt comprises a monovalentcation salt and/or a multivalent cation salt. The monovalent and/ormultivalent cation may comprise one or more of sodium, potassium,lithium, rubidium, cesium, ammonium, calcium, strontium, barium andmagnesium. In some embodiments, the alginate salt is sodium alginate,calcium alginate, or a mixture of sodium alginate and calcium alginate.

In certain embodiments, the stable or radioactive metal ion comprises agallium ion. Some embodiments provide that the gallium ion is onesuitable for imaging, such as ⁶⁶Ga, ⁶⁷Ga or ⁶⁸Ga. In other embodiments,the radioactive metal ion comprises one suitable for imaging such as⁶⁰Cu, ⁶¹Cu, ⁶²Cu, ⁶⁴Cu, ⁶⁷Cu, or ¹¹¹In.

In other embodiments, the alginate is conjugated to an iodinatedtyramine or tyramine derivative. In some embodiments, the iodinatetyramine or tyramine derivative may have the following formula:

-   -   wherein each X is independently hydrogen or iodine and R and R′        are each independently hydrogen, alkyl, benzyl or an aromatic        group. In some embodiments, the iodine is selected from the        isotopes of ¹²³I, ¹²⁴I, ¹²⁵I and ¹³¹I.

In some embodiments of this method, the mammal is a human.

Yet another aspect of the invention relates to a method of imagingalginate deposition in a mammal comprising administering a radio-labeledalginate conjugate and partially calcium-cross-linked alginate to amammal and imaging the radio-labeled alginate conjugate. In certainembodiments, the radio-labeled alginate conjugate is partiallycross-linked.

According to one or more embodiments of this aspect, the alginateconjugate has the formula:

-   -   wherein X is —(CH₂)_(m)C(O)—, with m being 1, 2, or 3;    -   L is a linker selected from the group consisting of:

-   -   wherein n, n′ and n″ are each independently a number from 0 to        10;    -   Alg is alginic acid or an alginate salt; and    -   Y is a spacer directly attached to the reducing end unit of        alginate or the carboxyl groups of the alginate polymer chain        via the active nitrogen, and selected from the group consisting        of:

-   -   wherein R is selected from the group consisting of hydrogen,        alkyl, benzyl or an aromatic group;

or a pharmaceutically acceptable salt or solvate thereof, and thealginate conjugate further comprises a stable or radioactive metal ionchelated by the 1,4,7-triazacyclononane-1,4,7-triacetic acid moiety ofthe conjugate.

In one or more embodiments, the alginate salt comprises a monovalentcation salt and/or a multivalent cation salt. The monovalent and/ormultivalent cation may comprise one or more of sodium, potassium,lithium, rubidium, cesium, ammonium, calcium, strontium, barium andmagnesium. In some embodiments, the alginate salt is sodium alginate,calcium alginate, or a mixture of sodium alginate and calcium alginate.

In certain embodiments, the stable or radioactive metal ion comprises agallium ion. Some embodiments provide that the gallium ion is onesuitable for imaging, such as ⁶⁶Ga, ⁶⁷Ga or ⁶⁸Ga. In other embodiments,the radioactive metal ion comprises one suitable for imaging such as⁶⁰Cu, ⁶¹Cu, ⁶²Cu, ⁶⁴Cu, ⁶⁷Cu, or ¹¹¹In.

In other embodiments, the alginate is conjugated to an iodinatedtyramine or tyramine derivative. In some embodiments, the iodinatetyramine or tyramine derivative may have the following formula:

-   -   wherein each X is independently hydrogen or iodine and R and R′        are each independently hydrogen, alkyl, benzyl or an aromatic        group. In some embodiments, the iodine is selected from the        isotopes of ¹²³I, ¹²⁴I, ¹²⁵I and ¹³¹I.

In some embodiments of this method, the mammal is a human.

The foregoing has outlined rather broadly certain features and technicaladvantages of the present invention. It should be appreciated by thoseskilled in the art that the specific embodiments disclosed may bereadily utilized as a basis for modifying or designing other structuresor processes within the scope present invention. It should also berealized by those skilled in the art that such equivalent constructionsdo not depart from the spirit and scope of the invention as set forth inthe appended claims.

DETAILED DESCRIPTION

Before describing several exemplary embodiments of the invention, it isto be understood that the invention is not limited to the details ofconstruction or process steps set forth in the following description.The invention is capable of other embodiments and of being practiced orbeing carried out in various ways.

Embodiments of the current invention provide for novel nuclear imagingreagents based on alginate conjugates for non-invasive clinical andpreclinical imaging of the heart and other organs and tissues. Suchimaging reagents are useful in measuring the kinetics of alginatedeposition in the injured myocardium and other bodily organs andtissues, and also may be useful to identify tissues or organs withsupraphysiological calcium concentration.

Sodium alginate is the structural polysaccharide that provides marineseaweed with its flexibility and strength. It is a linear binary blockcopolymer consisting of (1→4)-linked β-D-mannuronic and α-L-guluronicacid residues in various ratios and sequence arrangements. Calciumbinding is mediated by rigid homopolymeric guluronate sequences(G-blocks); the mannuronate blocks (M-blocks) do not bind calcium butprovide for flexible linkers connecting the calcium-binding G-blocks.According to the egg box model of calcium binding, one calcium ion isbound by two guluronate dimers located on two nearby alginate molecules,each dimer formed by two adjacent guluronate residues. Calcium bindingby G-blocks on alginate molecules is cooperative, leading to theformation of a zipper-like structure that crosslinks different alginatemolecules in a three-dimensional network. The crosslinking of alginatemolecules with calcium ions in aqueous solution induces the formation ofa mechanically resilient hydrogel that can be disassembled by removal orchelation of calcium.

To follow alginate hydrogel formation and deposition in mammals afterintracoronary injection, a labeling strategy for alginate was developedthat allows the imaging of alginate hydrogel formation and deposition byPET or SPECT. A radiometal approach was chosen over other radioligandmethodologies since (1) an unlabeled alginate conjugate precursor can beprepared in advance of the radioactive labeling; (2) the gellingproperties of the conjugate can be adjusted by choosing the extent andtype of substitution to mimic those of native alginate; (3) a properlydesigned conjugate can be labeled with a radioactive metal ion in a fastand mild binding reaction that is chemically compatible with alginate;(4) the radiometal-labeled conjugate can be quickly and quantitativelyseparated from free unbound radiometal; (5) the time required to preparethe radiometal-labeled alginate is short relative to the half-life ofthe radiometal. However, in some embodiments, the alginate isradiolabeled with other possible radioligands, such as iodinatedtyramine or tyramine derivatives.

In addition to the physiological crosslinking agent calcium, many otherdi- and trivalent cations of transition and heavy metals may bindalginate. Examples of such metals include Mg, Sr, Ba, Mn, Cu, Zn, Co,Cr, Al, Fe, Ga, In, Re, Pb, Hg and U. The direct metal binding byalginate has found many applications ranging from the removal of toxicand radioactive metal contaminants from drinking and waste water toimaging applications with the radiometal ¹¹¹In or imaging/radiotherapyapplications for ¹⁸⁸Re. The direct binding of a radiometal to alginatehas a number of disadvantages, however. For example, pathologicallyelevated tissue calcium may compete with the radiometal for alginatebinding and may induce the release of the metal ion from the alginatepolymer, this rendering the radiometal useless for tracking alginatedeposition. Also, the binding of the radiometal to the calcium bindingsites may alter the gelation properties of alginate which is undesirablefor an imaging reagent that is meant to mimic the deposition of alginatein a myocardial infarct.

Ligation of the radiometal to the alginate polymer via a bifunctionallinker molecule has potential several advantages over binding of theradiometal to the calcium binding sites of alginate: (1) the binding ofthe radiometal to a suitably engineered bifunctional linker can beengineered to be of high affinity while the affinity of direct metalbinding to the alginate polymer is low and is given by the structure ofalginate; (2) binding of calcium to alginate to induce gelation does notcompete for radiometal binding to the alginate conjugate, since calciumhas only low affinity for a properly designed bifunctional linker; (3)the radioactive concentration of the radiometal-labeled alginate can becontrolled by the ratio of chelator attached to alginate; and (4) thebinding of multivalent metals to alginate via a bifunctional chelatordoes not cause hydrogel formation by crosslinking of alginate molecules.

Sodium alginate can be chemically modified by attaching chemicalmoieties to the polymer. Examples include derivatization withbifunctional crosslinking agents to covalently crosslink alginate chainsinto a three-dimensional hydrogel that is independent of the calciumconcentration. Mono-functional reagents have been covalently bound toattach radioactive or histochemical labels that allow measuring alginatedeposition and degradation. Such chemical moieties can be attached toreactive groups present on the alginate polymer. Each hexuronic acidresidue of the alginate polymer has one carboxyl group and two hydroxylgroups that can be chemically modified. Moreover, each entire sodiumalginate molecule has one single reducing end to which one molecule canbe conjugated. In addition, reactive aldehydes can be generated alongthe polymer by gentle oxidation of the C2-C3 carbon-carbon bond.Derivatization chemistries for stable modification of alginate on thesesites include amide bond formation via activated ester, Schiff-baseformation and reductive amination, among others.

Many different types of metal chelators have been developed to bindradiometals to biological molecules. Examples of such chelators can befound in the review by Wadas et al. 2010 (Chem. Rev. 110: 2858-2902),which is hereby incorporated by reference in its entirety. The chelatorsdiffer in their chemical structure and their affinity and bindingkinetics for different metal ions and have different optimum bindingconditions required for metal complex formation. Many commonly usedchelators with high stability of the metal-chelator complex in vivorequire elevated temperature, e.g. 100° C., for efficient metal complexformation. Such elevated temperatures can be undesirable as they maydamage the macromolecule, leading to its degradation or denaturation. Asthe glycosidic bond of alginate may undergo hydrolysis at elevatedtemperatures, a chelator is preferred that avoids using hightemperatures for complex formation. The formed metal ion-chelatorcomplexes vary in their stability in biological fluids. Biological metalchelators in plasma such as albumin and transferrin may bind freeradiometals and will facilitate the disintegration of the metal-chelatorcomplex

Several different radiometals used in nuclear imaging can be used tolabeling alginate via a suitable metal chelator linked to alginate. Insome embodiments, the radiometal is gallium ion. Among the differentradiometals, gallium as trivalent Ga^(III) cation may be favored in someembodiments because (1) a single conjugate can be prepared for use withboth imaging modalities, PET and SPECT imaging, modalities withsufficient sensitivity, spatial and temporal resolution as galliumisotopes suitable for both imaging modalities exist; (2) the macrocyclicgallium ion chelator 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA)forms a high affinity Ga^(III)-NOTA complex with gallium within minutesat physiological temperatures (20° C. to 37° C.); this complex is stablein blood; (3) the affinity of NOTA for gallium ion is high enough toavoid significant gallium binding to the calcium binding sites on thealginate polymer during the labeling reaction; and (4) the chelator NOTAcan be chemically modified with a spacer molecule to form a bifunctionallinker that attaches NOTA to one of the reactive sites of alginatewithout significantly altering the chelator's binding affinity forgallium ion. However, other suitable radiometals or radioligands asdescribed herein may also be used for radiolabeling the alginate.

Published studies document the binding of gallium to an alginatebioglass biomaterial with the purpose of providing for an antibacterialyet tissue-compatible biomaterial conducive for bone regeneration. Thismaterial differs from the gallium-NOTA alginate conjugate described herein that it does not employ a bifunctional linker to enable stable andhigh-affinity binding of gallium to the alginate polymer but utilizesthe weak and reversible binding of gallium ion to the alginate polymerto create a gallium ion-releasing antibacterial scaffold for bone tissueengineering.

Thus the labeling of alginate with gallium metal ion via a bifunctionallinker that binds gallium ion and attaches covalently to alginate, thechemical synthesis of a NOTA-containing alginate conjugate, thepreparation of a gallium NOTA alginate conjugate and the use of such agallium-labeled NOTA-alginate conjugate in preclinical and clinicalimaging applications are novel and have not been previously reported.

Although specific reference is made to sodium alginate, other alginatesare compatible with embodiments of all the aspects described herein.Alginates from different algae and bacterial strains vary in theirmolecular weight, polydispersity, guluronic acid content, sequencearrangement of G-blocks, M-blocks and GM-blocks, and chemicalmodification. These differences do not alter their suitability for thedescribed imaging application. Alginates can be fractionated bymolecular weight and chemical composition, chemically derivatized orfunctionalized with cross-linkers or other reagents and prepared indifferent salt forms including the free acid, neither of which wouldrender the described imaging strategy impossible to pursue.

According to one aspect of the present invention, provided arebifunctional linkers derived from the structure of NOTA having thegeneral structure of Formula (I):

wherein X is —(CH₂)_(m)C(O)—; m is 1, 2, or 3; L is a linker; H ishydrogen; and Y—H is a functional group capable of conjugating withalginate via direct attachment or reductive amination to the reducingend, or via amide bond formation with the carboxyl groups on the chainof alginate. In one or more embodiments, the compound is apharmaceutically acceptable salt or solvate of the compound of Formula(I).

These NOTA-derived linkers are bifunctional molecules that after theirconjugation to alginate allow for the labeling of alginate with metalions. The metal ions can be stable or undergo radioactive decay. In someembodiments, the NOTA-derived linker is labeled with a radioactive metalion. In some embodiments, the NOTA-derived linker is labeled with ⁶⁶Ga,⁶⁷Ga or ⁶⁸Ga. In a particular embodiment, the radioactive metal ion isthe gamma-ray emitting ⁶⁷Ga isotope. In other embodiments, theradioactive metal ion is a positron-emitting ⁶⁶Ga or ⁶⁸Ga isotope.

In one or more embodiments, linker L has one of the followingstructures:

-   -   wherein n, n′ and n″ are each independently a number from 0 to        10.

According to one or more embodiments, Y—H is selected from the groupconsisting of:

-   -   wherein R is selected from the group consisting of hydrogen,        alkyl, benzyl or an aromatic group.

Another aspect of the invention relates to a metal-chelator-containingalginate conjugate. In one exemplary embodiment, this conjugate is aNOTA-alginate conjugate having the general structure of Formula (II):

-   -   wherein X is —(CH₂)_(m)C(O)—; m is 1, 2, or 3; L is a linker;        Alg is alginic acid or an alginate salt; Y is a spacer directly        attached by reductive amination to the reducing end, or via        amide bond formation to the carboxyl groups of the alginate        polymer. The molar ratio of NOTA-linker to alginate can be        controlled by the conjugation reaction conditions to suit        particular needs of the imaging application. In one or more        embodiments, the compound is a pharmaceutically acceptable salt        or solvate of the compound of Formula (II).

These NOTA-alginate conjugates can be labeled with metal ions. In someembodiments, the NOTA-alginate conjugate is labeled with a radioactivemetal ion. In further embodiments, the NOTA-alginate conjugate islabeled with gallium isotopes suitable for imaging applications such as⁶⁶Ga, ⁶⁷Ga or ⁶⁸Ga. In a preferred embodiment, the radioactive metal ionis the gamma-emitting isotope ⁶⁷Ga. In an alternate embodiment, theradioactive metal ion is the positron-emitting ⁶⁶Ga or ⁶⁸Ga isotope.

In one or more embodiments, L has one of the following structures:

-   -   wherein n, n′ and n″ are each independently a number from 0 to        10.

In one or more embodiments, the alginate salt comprises a monovalentcation salt and/or a multivalent cation salt. The monovalent and/ormultivalent cation may comprise one or more of sodium, potassium,lithium, rubidium, cesium, ammonium, calcium, strontium, barium andmagnesium. In some embodiments, the alginate salt is sodium alginate,calcium alginate, or a mixture of sodium alginate and calcium alginate.

In certain embodiments, the alginate has a molecular weight betweenabout 10 and about 100 kDa. In a specific embodiment, the alginate has amolecular weight of about 30 kDa.

According to one or more embodiments, Y has one of the followingstructures:

-   -   wherein R is selected from the group consisting of hydrogen,        alkyl, benzyl or an aromatic group.

Another aspect of the invention pertains to the use of these conjugatesin clinical imaging. Examples of such imaging modalities include, butare not limited to PET or SPECT. Other types of imaging can be used withthese radiometal-labeled alginate conjugates. For example,radiometal-labeled alginate conjugates can be employed in imaging bywhole-body autoradiography in preclinical settings or with a gammacamera (Anger camera) in clinical or preclinical settings. Such imagingcan provide non-invasive monitoring of aqueous cross-linked alginatesolution in the bodies of mammals, as well as the visualization of itscardiac deposition. In one or more embodiments, the mammal is a human.

The NOTA-linker or the NOTA-alginate conjugates can be labeled withmetal ions other than gallium. Metal ions with applications in nuclearimaging and high affinity for NOTA include copper and indium, inparticular the copper isotopes ⁶⁰Cu, ⁶¹Cu, ⁶²Cu , ⁶⁴Cu, and ⁶⁷Cu or theindium isotope ¹¹¹In.

Other radio-labeled alginate conjugates may also be used. For example,the alginate may be conjugated to an iodinated tyramine or tyraminederivative. In some embodiments, the iodinate tyramine or tyraminederivative may have the following formula:

-   -   wherein each X is independently hydrogen or iodine and R and R′        are each independently hydrogen, alkyl, benzyl or an aromatic        group. In some embodiments, the iodine is selected from the        isotopes of ¹²³I, ¹²⁴I, ¹²⁵I and ¹³¹I.

⁶⁸Ga has many potential advantages for clinical PET imaging. Unlikeother positron emitters (e.g. ¹⁸F, ⁶⁴Cu, and ¹²⁴I), ⁶⁸Ga (T_(1/2)=68min, β⁺=89% and EC=11%) can be produced by use of a commerciallyavailable ⁶⁸Ge/⁶⁸Ga generator. Likewise, ⁶⁷Ga has significant advantagesfor SPECT imaging as it is approved and commercially available forclinical use as the citrate salt. The high specific activity of eitherisotope allows the production of gallium-labeled NOTA-alginate conjugatewith high specific activity suitable for SPECT imaging when used eitheras a partially cross-linked gallium NOTA alginate conjugate formulationor as a tracer diluted in to a partially cross-linked alginate solution.

An advantage of gallium over other metal ions is that gallium ion formsa very stable complex with derivatives of1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) at room temperatureor mammalian body temperature. Although sodium alginate may bind galliumion, gallium binds to NOTA with higher affinity and the Ga^(III)-NOTAcomplex is stable even in the presence of 100× molar excess of sodiumalginate at 37° C. Thus, a NOTA-alginate conjugate can be successfullylabeled with stable gallium or radioactive gallium isotopes such as⁶⁶Ga, ⁶⁷Ga and ⁶⁸Ga.

The gallium-labeled NOTA-alginate conjugate can be formulated by partialcrosslinking with calcium ions for preclinical and clinical use in twodifferent formats.

(1) The gallium-labeled NOTA-alginate conjugate can be formulated as apartially calcium-cross-linked homogeneous solution in which themajority or all alginate molecules are derivatized with NOTA-linker andat least part of all NOTA linker moieties is chelated with radiogallium.Because of the large abundance of NOTA sites in this preparation, thelabeling of NOTA with gallium ion can be highly sub-stoichiometric, withgallium bound to only a small fraction of NOTA sites while leaving themajority of all NOTA sites free of gallium ion. To use this material forthe imaging of cross-linked alginate deposition, an amount of formulatedgallium-labeled NOTA-alginate conjugate similar to that of formulatedpartially calcium-cross-linked alginate is administered by intracoronaryinjection. Thus in this format, the partially calcium cross-linkedgallium-labeled NOTA-alginate is used instead of and replaces thepartially calcium cross-linked alginate.

(2) Alternatively, gallium-labeled NOTA-alginate may be synthesized thatcontains a high proportion of NOTA sites labeled with gallium ion. Thismaterial has sufficiently high specific activity such that suchgallium-labeled NOTA alginate conjugate can be used as a tracer bymixing small quantities of the gallium-labeled NOTA alginate conjugatewith partially calcium-cross-linked alginate solution for intracoronaryinjection. When used in this manner, the deposition of gallium-labeledNOTA-alginate conjugate tracks the deposition of partiallycalcium-cross-linked alginate in the heart tissue by means of itsincorporation into the calcium-cross-linked alginate hydrogel.

EXAMPLES

Non-limiting examples of the compounds of the present invention havebeen synthesized (Examples 1-5), and all intermediates and finalproducts were characterized by ¹H-NMR, LC-MS and/or elemental analysis).Conditions for labeling NOTA-alginate conjugate with radiogallium areshown in Scheme 6.

Example 1 Synthesis of Tyramine Derivative of NOTA

A tyramine derivative of NOTA was synthesized according to Scheme 1:

Example 2 Synthesis of NOTA-Alginate Conjugate from Tyramine Derivativeof NOTA

The tyramine derivative of NOTA from Example 1 was reacted with sodiumalginate in the presence of4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride(DMTMM) and water. Scheme 2 shows the formation of an amide bond betweenthe amino group of the tyramine derivative of NOTA and the carboxylgroup of the alginate:

Example 3 Alternative Synthesis of NOTA-Alginate Conjugate from TyramineDerivative of NOTA

An alternative to the reaction shown in Scheme 2 is shown in Scheme 3.The tyramine derivative of NOTA from Example 1 was reacted with alginicacid under the conditions as shown by Scheme 3. Instead of forming anamide bond as in Example 2, reductive amination takes place between theamine in the tyramine derivative of NOTA and the reducing end unit ofthe alginate.

Example 4 Synthesis of N-methylhydroxylamine-Containing NOTA

An N-methylhydroxylamine-containing NOTA linker was synthesizedaccording to Scheme 4:

Example 5 Synthesis of NOTA-Alginate Conjugate fromN-methylhydroxylamine-Containing NOTA

As shown in Scheme 5, a NOTA-alginate conjugate was produced by directlyattaching the N-methylhydroxylamine-containing NOTA linker from Example4 to the reducing end unit of alginate:

Example 6 Labeling of NOTA-Alginate Conjugate

Scheme 6 shows labeling conditions for NOTA-alginate conjugate: (1)incubation for 15 min at 37° C. of NOTA-alginate conjugate with ⁶⁸GaCl₃eluted from a generator, or ⁶⁷GaCl₃ from a cyclotron, (2) rapidseparation by gel filtration or molecular weight cutoff filtration ofthe radio-labeled conjugate from unbound free gallium ion, and (3)either mixing of the gallium-labeled NOTA linker alginate conjugate withpartially cross-linked calcium alginate solution for injection orformulation with calcium ions to produce a partially cross-linkedgallium-labeled NOTA alginate conjugate.

Reference throughout this specification to “one embodiment,” “certainembodiments,” “one or more embodiments” or “an embodiment” means that aparticular feature, structure, material, or characteristic described inconnection with the embodiment is included in at least one embodiment ofthe invention. Thus, the appearances of the phrases such as “in one ormore embodiments,” “in certain embodiments,” “in one embodiment” or “inan embodiment” in various places throughout this specification are notnecessarily referring to the same embodiment of the invention.Furthermore, the particular features, structures, materials, orcharacteristics may be combined in any suitable manner in one or moreembodiments.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It will be apparent to those skilled in the art thatvarious modifications and variations can be made to the method andapparatus of the present invention without departing from the spirit andscope of the invention. Thus, it is intended that the present inventioninclude modifications and variations that are within the scope of theappended claims and their equivalents.

What is claimed is:
 1. An alginate conjugate having the formula:

wherein X is —(CH₂)_(m)C(O)—, with m being 1, 2, or 3; L is a linkerselected from the group consisting of:

wherein n, n′ and n″ are each independently a number from 0 to 10; Algis alginic acid or an alginate salt; and Y is a spacer directly attachedto the reducing end unit of alginate or the carboxyl groups of thealginate polymer chain via the active nitrogen, and has the followingstructure:

wherein R is selected from the group consisting of hydrogen, alkyl,benzyl or an aromatic group; or a pharmaceutically acceptable salt orsolvate thereof.
 2. The alginate conjugate of claim 1, wherein theconjugate further comprises a stable or radioactive metal ion chelatedby the 1,4,7-triazacyclononane-1,4,7-triacetic acid moiety of theconjugate.
 3. The alginate conjugate of claim 2, wherein the stable orradioactive metal ion comprises a gallium ion.
 4. The alginate conjugateof claim 2, wherein the radioactive metal ion comprises ⁶⁰Cu, ⁶¹Cu,⁶²Cu, ⁶⁴Cu, ⁶⁷Cu, or ¹¹¹In.
 5. A method of imaging in a mammalcomprising: administering the alginate conjugate of claim 1 to a mammal;and imaging the temporal and spatial distribution of the alginateconjugate.
 6. The method of claim 5, wherein the alginate conjugatefurther comprises a stable or radioactive metal ion chelated by the1,4,7-triazacyclononane-1,4,7triacetic acid moiety of the conjugate. 7.The method of claim 6, wherein the stable or radioactive metal ioncomprises a gallium ion.
 8. The method of claim 6, wherein theradioactive metal ion comprises ⁶⁰Cu, ⁶¹Cu, ⁶²Cu, ⁶⁴Cu, ⁶⁷Cu, or ¹¹¹In.9. The method of claim 5, wherein the alginate conjugated is partiallycross-linked.
 10. A method of imaging alginate deposition in a mammalcomprising: administering a mixture of and the alginate conjugate ofclaim 1 and partially calcium-cross-linked alginate to a mammal; andimaging the temporal and spatial distribution of the alginate conjugate.11. The method of claim 10, wherein the alginate conjugate furthercomprises a stable or radioactive metal ion chelated by the1,4,7-triazacyclononane-1,4,7-triacetic acid moiety.
 12. The method ofclaim 11, wherein the stable or radioactive metal ion comprises agallium ion.
 13. The method of claim 11, wherein the radioactive metalion comprises ⁶⁰Cu, ⁶¹Cu, ⁶²Cu, ⁶⁴Cu, ⁶⁷Cu, or ¹¹¹In.
 14. The method ofclaim 10, wherein alginate conjugate is partially cross-linked.